Multiple frequency bands for wireless communication have been used in smart phones or tablets. For example, a smart phone could include an RF frequency band to cover such services as GSM, GPS, WiFi, Bluetooth, 3G or 4G LTE. A compact bandpass RF/microwave filter which can cover multiple working frequency bands is an essential component for enabling a more integrated solution for these communication handheld devices.
WiMax has also attracted much attention for long-range wireless network, especially for broadband wireless access in the frequencies from 2 to 11 GHz supported by the IEEE802.16 standard. A compact bandpass RF/microwave filter which can cover ultra wide working frequency range from 2 to 11 GHz is an essential component for WiMax communication handheld devices.
In order to achieve a wide working frequency range, different kinds of filters have been proposed and studied intensively. For planar microstrip filters, working frequencies of 4.14-6.26 GHz and 1.178-3.6 GHz have been demonstrated by filters with footprints of about 17×20 mm2 and 13×18 mm2 respectively. However, due to the utilization of quarter wavelength resonators, a planar microstrip filter doesn't have the capability of covering a very wide working frequency range. In contrast, capacitive-post loaded evanescent mode cavities were demonstrated to be capable of having a wider working frequency of 0.98-3.48 GHz and 1.9-5 GHz by sizes of 41.5×24.9×3.17 mm3 and 30.0×18.0×4.5 mm3 respectively. Furthermore, when micro-fabricated in Silicon, such a filter design showed a size of 10.0×5.0×2.5 mm3 and working frequency of 6 to 24 GHz.
However, its return loss and insertion loss was not satisfactory due to the limitation of a relatively narrow band energy coupling capability of the cavity structure.
Electromagnetic Bandgap (EBG) filters have also emerged as an alternative design attributed to their high Qu, ease of integration and low cost. However, these proposed EBG filters/resonators were implemented by modification of the periodic lattice in the EBG substrate which limits their use for broad band coverage.
Further, more wireless systems are calling for multifunctional or multiband operations with the support of a single broadband transceiver module. A multiband bandpass filter with compact size, planar configuration, and high performance is an integral part of a single transceiver implementation architecture.
Multiband filters have been researched extensively and their implementations can be classified into three main categories. The first category includes using two or more resonators with controllable fundamental and higher order resonant modes, such as stepped impedance resonators (SIR) in dual band bandpass filters (BPF), stub loaded SIR in dual band BPF, stub loaded resonator in dual band BPFs, and quad mode resonators in quad band BPFs. Generally, the nth resonant modes of each resonator need appropriate coupling for building up the nth pass band. Since the resonant modes of such resonators are often dependent on each other, this method sometimes is difficult to place some resonant modes in desired or useful frequencies.
The second category includes the dual mode multiple band BPF for multiple band applications. This candidate has been attractive because of its compact size, simple physical layout, and design procedure. However, the dual mode multiple band BPFs using a single resonator have reported a poor band to band isolation and notch like upper stopband of the second pass band.
The third category of multiband filters includes multiple independently constructed single band filters working at specifically selected frequencies combined to implement a multiple band filter by sharing input and output ports. A good isolation between those bands has been achieved, however, the filters usually have an area multiple times larger than that of the single band filters.